The present invention generally relates to a method and apparatus to improve the appearance of images produced by an output device. More particularly, the present invention is directed to an adaptive image enhancement filter for generating enhanced print data to drive an image output terminal.
A common goal in the development of image output systems is improving image quality. One method of achieving improved image quality in digital image output systems is through the use of image enhancement filters. Image enhancement filters can be used to perform a variety of image processing operations including resolution conversion, enhancement, rotation, restoration, appearance matching or tuning, and de-screening for both binary and grayscale images.
Image enhancement filters frequently are achieved using template-based or template-matching filters which are implemented using look-up tables due to their design versatility and ease of implementation and use. Each filter includes a number of template operators, or simply templates, to filter or transform an input image into an enhanced output image by, in a typical image processing setting, transforming certain observed pixel patterns in the input image into corresponding enhanced pixel pattems. Specifically, the filter receives an arrangement of pixels identified by a window of predetermined size and shape defining a set of pixels located about a target pixel. The enhancement filter compares the received pixel pattern defined by the window to one or more templates to determine if there is a match. If a match to the observed pixel pattern is found, the filter generates an appropriate output signal such as an enhanced pixel pattem for the target pixel. This output signal can be used to drive the image output device or be used as an input to further image processing operations. To process a given region of an image, the window is typically scanned across an image advancing from target pixel to target pixel in a predetermined sequence.
The design of an image enhancement filter for generating an enhanced output signal to drive an image output device requires the identification of the image structures that should be enhanced as well as knowledge of the marking process characteristics. Identification of the image structures that should be enhanced enables a determination of the matching templates required for the filter. Knowledge of the marking process and its characteristics is essential for the development of an image enhancement filter. The signals employed to drive an image output terminal as well as the marking process used to generate the output image have an effect within the output image that may be thought of as the amount darkness generated, the resulting shift in an edge, the resulting line thickness, or the resulting pixel density. The possible output signals can be calibrated so as to characterize the effect they generate, and then, knowing their effect, an appropriate signal can be identified for each image structure setting.
Typically, image enhancement filters are designed to output a specific enhanced signal for each one of a predetermined set of templates. While such image enhancement filters have proven to be a very effective tool for improving image quality, existing methods and devices for implementing the filters often suffer from one or more drawbacks which may limit their effectiveness in some applications. One such drawback of conventional image enhancement filters is the inability to easily or automatically adapt the filter for marking processes with varying characteristics. Consider, for example, an image enhancement filter for generating an enhanced image signal for driving an image output terminal. As described above, the enhanced signal for a given template will be based, in large part, on the characteristics of the marking process. Any variations in the marking process or the process characteristics may have an impact on the enhanced signal needed to accurately reproduce the input image. However, existing image enhancement filters do not permit these variations in the marking process characteristics to be addressed without loading and implementing a new filter.
One aspect of the present invention is an adaptive image enhancement filter for receiving image data including a plurality of pixels and generating enhanced image data for driving a marking engine for each pixel within the image data. The adaptive image enhancement filter includes a template matching processor that receives an observation pattern including a target pixel and at least one pixel neighboring the target pixel. The template matching module compares the received observation pattern to a set of templates and generates a match identification signal. A signal generation processor connected to receive the match identification signal and a marking process attribute signal identifies a set of optimized drive signals in response to the marking engine attribute signal and in response to the received match identifier provides optimized drive signals from the identified set of drive signals as the enhanced image data for the target pixel.
Another aspect of the present invention is a method of generating enhanced image data for driving a marking engine. The method includes the steps of (a) receiving a set of pixels to be printed, the set of pixels including an observation window identifying a target pixel and at least one neighboring pixel; (b) comparing the pixels within the observation window to a set of pixel templates; (c) generating a template match identifier in response to the comparing step; (d) selecting one of a plurality of sets of optimized drive signals, each set of optimized drive signals containing a plurality enhanced signals for driving the marking engine; and (e) using the match identifier select one of enhanced signals from the selected set of optimized drive signals as the enhanced image data.